This invention relates to polymer polyol compositions useful as intermediates in the preparation of polyurethanes. More specifically, this invention pertains to polymer polyol compositions containing grafted polyol-polyacrylate dispersants. The grafted polyol-polyacrylate dispersants may be obtained by copolymerizing isocyanate vinyl monomers and acrylate monomers and then reacting the resulting copolymers with polyoxyalkylene polyols.
In one of its more specific aspects, this invention relates to the incorporation of grafted polyol-polyacrylate dispersants into polymer polyol compositions for the purpose of improving the stability of the dispersed vinyl polymer particles contained in the polymer polyol compositions. The stabilized polymer polyol compositions have relatively low viscosity and small average particle size.
Polymer polyols containing vinyl polymer particles dispersed in a continuous polyoxyalkylene polyol phase are well known. Also well known is the tendency of polymer polyols to undergo phase separation if they are not stabilized. Hence, polymer polyols are commonly stabilized against phase separation by the addition of a polymeric dispersant during preparation of the dispersion.
U.S. Pat. No. 4,745,153 teaches a dispersant formed by copolymerizing a vinyl-terminated polyol adduct with an ethylenically unsaturated monomer. A polymer polyol is then prepared by reacting an ethylenically unsaturated monomer, the dispersant, and a base polyol under free radical polymerization conditions.
U.S. Pat. Nos. 4,390,645 and 4,460,715 teach the use of a preformed dispersant prepared by reacting an ethylenically unsaturated isocyanate with a polyoxyalkylene polyol using a very low NCO: active hydrogen molar ratio, and then copolymerizing the monovinyl adduct thus obtained with a styrene/acrylonitrile mixture.
U.S. Pat. Nos. 4,148,840 and 4,242,249 teach the use of certain preformed polymer polyols as suitable dispersant stabilizers. U.S. Pat. Nos. 4,327,005 and 4,334,049 teach that alkylene oxide adducts of styrene/allyl alcohol copolymers can be used as polymer polyol dispersants.
Although polymer polyols prepared by prior art methods have enjoyed widespread commercial use in the polyurethane industry due to the beneficial properties which they impart to polyurethane foams, the need for further improvement in certain properties of the available polymer polyols has been recognized. Improved stability is desirable since the time period during which a polymer polyol can be stored before significant settling of the dispersed vinyl polymer particles occurs would thereby be increased. In addition, lowering the viscosity of a polymer polyol composition would reduce the handling problems encountered during processing and use of the composition. Small particle size is also desirable since plugging and fouling of processing equipment would be minimized.
Polymer polyols are commonly prepared using a mixture of styrene and acrylonitrile to form the dispersed vinyl polymer particles. It is widely recognized that increasing the ratio of styrene to acrylonitrile is beneficial since the substitution of styrene for acrylonitrile helps prevent discoloration when the polyurethane is cured and also improves the flame retardant properties of polyurethane products. However, this substitution is typically difficult to accomplish since the stability of the polymer polyols decreases with increasing styrene to acrylonitrile ratios. Viscosity and particle size are also often adversely affected by high styrene content.
Increasing the concentration of vinyl polymer dispersed in a polymer polyol is acknowledged to result in certain improvements in polyurethane properties, most notably the load bearing properties. Furthermore, the production of a polymer polyol having a high vinyl polymer level is economically attractive since the resulting product can be readily diluted after preparation to a lower solids level with additional base polyol. Thus, a larger volume of polymer polyol containing a conventional amount of dispersed vinyl polymer can be produced using existing equipment. However, increasing the vinyl polymer content of a polymer polyol generally leads to increased viscosity and lower dispersion stability using prior art methods.
Clearly, there is a need for improved dispersants capable of providing stable, low viscosity polymer-polyol compositions especially when such compositions have high concentrations of dispersed solids or high styrene/acrylonitrile ratios.